Organic light-emitting devices, for example organic light emitting diodes (OLEDs), are broadly researched for their application in flat-panel displays. Unlike liquid crystal displays, flat-panel displays employing organic light-emitting devices are brighter, because organic light-emitting devices can emit light themselves and do not need backlight systems. In addition, with different organic materials, organic light-emitting devices can emit light in red, green, and blue colors with high luminance efficiency. Moreover, organic light-emitting devices can operate with low driving voltages and are viewable from oblique angles.
Organic light-emitting devices are usually structured to have several layers, including at least a layer of organic light-emitting material sandwiched between an anode and a cathode. The basic principle of operation for an organic light-emitting device is that, when a voltage is applied between the anode and cathode, electrons and holes are driven to move to the layer containing organic light-emitting material. Electrons and holes meet and emit light.
U.S. Pat. Nos. 6,137,223, 6,579,629, and 6,013,384 are expressly incorporated by reference herein in their entireties, for their teachings on OLEDs. The anode is typically made of light transmitting material such as indium tin oxide (ITO) and the cathode is made of metal. After the deposition of the organic light-emitting layer, the cathode is formed above the organic light-emitting layer. Among methods to deposit the cathode, thermal evaporation and electron beam deposition are preferred because they do not damage the organic light-emitting layer. However, these processes are very inefficient. On the other hand, sputtering has many advantages such as short cycle time, adherent coating, and economical use of deposition materials, although it can significantly damage the electrical characteristics of the organic light-emitting layer. A structure is desired for organic light-emitting devices having a cathode that can be deposited by sputtering without substantial damage to the organic light-emitting layer.